1. Field of the Invention
This invention relates to an electromechanical transducer and, more particularly, to a transducer commonly known as a mass loaded longitudinal vibrator transducer having the dominant mechanical motion in a single direction and which includes head masses alternating with compliant layers where one of the compliant layers can be an active transducer element.
2. Description of the Related Art
A device commonly known as a "longitudinal vibrator" is one of the simplest and most widely used electromechanical or electroacoustical transducer types. Such a device, in its simplest form, consists merely of a thin piece of active material which can be driven electrically to induce a planar motion therein. For example, a flat disc or ring made of a piezoelectric ceramic (such as a lead zirconate titanate formulation) which has electrodes on its flat surfaces and is polarized in the direction normal to the flat surfaces may act as a vibrator. This type of device is usually operated at its first longitudinal resonance frequency to achieve a higher output. To achieve a reasonably low resonance frequency and a well controlled response in a compact device, it is common practice to mass load the two sides of the active material with inactive material pieces.
An example of a prior art double mass loaded longitudinal vibrator is shown in FIG. 1(a). Piezoelectric material rings 1 are bonded into a composite stack 2 and electrically wired in parallel so that when a voltage is applied between the electrical leads, all of the rings expand or contract in unison along with longitudinal axis of the device. A single vibrating head mass element 3 having a forward face 4 acts as the radiating surface and as a load on the forward end of the stack. Tail mass 5 is attached to the other end of the stack and is normally higher in mass than the head 3 to cause the predominant motion to be in the head 3. A stress rod or pretension bolt 6 and an associated nut 7 are used to join the components and to provide a compressive bias to the stack 2 of active elements.
The device of FIG. 1(a) may be used as either a generator or receiver of mechanical or acoustic energy and is normally operated in a frequency band approximately centered on its primary mechanical resonance frequency. At the primary resonance frequency, the head mass 3 and tail mass 5 move in opposite relative directions while the stack of active material alternately expands and contracts along its length.
It is well known by those of ordinary skill in the art that the performance of the conventional transducer in FIG. 1(a) can be approximated by the analogous behavior of a simplified electrical equivalent circuit, as shown in FIG. 1(b). In the circuit, M.sub.h and M.sub.t represent the head and tail mass, respectively. The transformer represents the electromechanical transformation property of the piezoelectric material. The turns ratio .phi. of this transformer is the electromechanical transformation ratio. The compliance of the ceramic stack 2 is represented by the capacitor C, and C.sub.0 represents the actual electrical capacitance of the stack. The electrical components to the right of the transformer represent mechanical components while those to the left represent actual electrical components. The box at the right of the equivalent circuit represents the electrical equivalent of the radiation impedance Z.sub.rad seen by the radiating face of the transducer. The equivalent current u in the radiation impedance represents the velocity of the moving face of the radiator.
The transmitting voltage response (TVR) of this prior art device is calculated from this equivalent circuit approximation and is proportional to the current u divided by the drive voltage E at the input to the transducer circuit. In determining the response of the device, as expressed by Equation (1) below, the radiator impedance can be neglected. ##EQU1## The transmitting voltage response has a single peak near the frequency where the denominator of the expression becomes zero. This occurs at the resonance (angular) frequency w.sub.r as set forth in Equation 2 below: ##EQU2## The method of analysis discussed above is well known in the transducer industry, as discussed in, for example, Leon Camp, Underwear Acoustics, Wiley & Sons, New York, 1970, pp. 142-150; and Berlincourt et al, "Piezoelectric and Piezoelectric Materials and Their Function and Transducers", Physical Acoustics, Vol. 1A, Academic Press, New York, 1964, pp. 246-253. More complete and accurate performance predictions for transducers can be obtained by using a computer model, such as developed by K. M. Farnham, obtainable from Transducer and Arrays Division, Naval Underwater Systems Center, New London Laboratory, in New London, Conn. A graph of a typical response curve, produced by the above-mentioned program, for the transducer of FIG. 1(a) is illustrated by curve 30 in FIG. 6. The bandwidth of such a device is 0.36 frequency units extending from 0.85 to 1.21 frequency units.
A significant drawback of the prior art transducer of FIG. 1(a) is the very low mechanical input impedance of the radiator face (which is hereafter called the head impedance) in the operating band at the resonant frequency. This low head impedance can cause problems when the transducer is used as an element in array configuration. As a practical limit, it is often necessary that the head impedance for elements in a high performance array be maintained at a magnitude sufficiently larger than the acoustic mutual impedances of the array for all possible operating frequencies. For this reason, it is often necessary to preclude operation in a narrow frequency band near the peak of the response.
The basic device, as shown in FIG. 1(a) also has significant practical limits to the achievable frequency bandwidth. The operating bandwidth can be increased by simultaneously reducing the mass of the head section and increasing the mechanical compliance of the ceramic stack. This results in a thinner head section and a ceramic stack which is smaller in cross-sectional area and/or greater in length. However, this design technique is limited by the following practical design considerations. As the radiating face becomes thinner, its first flexural resonance frequency will encroach on the operating band and drastically alter the radiation behavior of the unit. As the active material becomes thinner and/or longer, the device becomes mechanically fragile. As the active material becomes longer to maintain the desired resonance frequency, its length will become a significant fraction of the mechanical wavelength in the material, at which point the material is self-resonant and does not drive the medium.
In an effort to broaden the operating bandwidth of mass loaded longitudinal vibrators, a number of techniques have been attempted. One technique uses electrical components, such as inductors or capacitors, connected between the electrical terminals of the transducer and associated amplifier circuits to tune the response of the device. However, the modification using the special electrical termination can expand the bandwidth to a limited extent at the cost of increased size, weight and complexity. In addition, this method may produce localized high voltages at some circuit nodes requiring costly high voltage isolation and shielding. Curve 31 in FIG. 6 illustrates the response which is typical when a transducer is operated with an inductor in series with its electrical leads. As with the untuned transducer, the tuned transducer when operated in an array configuration encounters significant practical problems in the frequency range near each of the response peaks. However, this design offers a significant bandwidth between the peaks which is free from array problems caused by low head impedance. The 3 dB bandwidth around the dip between the two peaks extends from 0.81 to 1.20 relative frequency units and has a width of 0.39 frequency units.
Another well known technique for broadening the operating bandwidth of a transducer is to use external matching layers. The acoustic impedance of the transducer and the medium are matched through an external matching layer 8 as illustrated in FIG. 1(a) by the dashed lines. A design incorporating an external matching layer can achieve a significantly increased bandwidth as seen in curve 32 of FIG. 6. However, there are disadvantages to this approach. With external matching layers, the shape of the response curve is a fairly sensitive function of the density and sound speed of the matching layer material. In a particular application, it may not be possible to find an acceptable material having the desired material properties. In addition, the required matching layer thickness may be greater than is desired. Furthermore, in a design which incorporates an external matching layer, there will be two frequencies at which the head impedance becomes unacceptably low for operation in an array. For example, the transducer whose response is shown in curve 32 has an external matching layer of lucite and head impedance dips at relative frequencies of 0.80 and 1.78. Array operation would be inappropriate at either of these frequencies. Thus, the response shown has a useful bandwidth extending from 0.85 to 1.73 or 0.88 frequency units. If the head impedance dips can be tolerated, then the 3 dB bandwidth extends from 0.78 to 1.90 frequency units, a bandwidth of 1.12 units.
Another approach used in obtaining two resonant frequencies in a transducer is illustrated in FIG. 2. This device consists of a low frequency vibrator 10, a head mass 11 and a high frequency nonlinear array 12 nodally mounted on the head mass 11. In addition to being an extremely complex device requiring complex wiring and mounting systems, the device of FIG. 2 provides high power transmission in two widely spaced frequency bands more than two octaves apart. Each of the transducers 10 and 12 in this device operate as if the other were not present. However, the transducer of FIG. 2 cannot provide broadband response between the two resonance frequencies because there is an additional resonance in the transducer caused by the nodal mounting system of the high frequency units 12 on the head mass 11 of the low frequency unit 10. The resonance frequency of the nodal mounting system must occur at a frequency very much lower than the high frequency operating resonance to decouple the performance of the high frequency unit from the structure of the low frequency unit. In addition, the nodal mounting resonance frequency must be above the low frequency operating band or else it would act to decouple the low frequency radiator from the radiating medium. The acoustic reponse of the transducer undergoes a drastic decrease in the frequency range of the mounting system resonance. As a consequence, the prior art device of FIG. 2 cannot be used as a broadband transducer throughout the frequency range between the high and low frequency resonances because the nodal mounting resonance frequency must be between the two operating bands of the device. Thus, this device does not provide a true broadband transducer, but two separate frequency bands.
Other features of typical transducers, such as insulating washers, wiring, electrical contacts, etc., are well known by those of ordinary skill in the art and can be found in, for example, U.S. Pat. No. 3,309,654 to Miller.